The present invention relates to correction of East-West distortion of images produced on the screen of a picture tube. More particularly, it concerns an electronic circuit for controlling the AC component of the drive signal to an East-West amplifier in a picture tube horizontal scanning system.
The proposed circuit seeks to improve an existing East-West correction system for the horizontal scanning function of a television or monitor picture tube. The known system comprises a diode modulator which varies the scan current to the deflection coils of the picture tube in response to a voltage drive from an East-West amplifier, without significantly affecting the secondary waveforms and supplies generated by the horizontal output stage.
The output of the East-West amplifier has a DC component which determines the line scan width, and an AC (parabolic) component that corrects an inherent pincushion distortion of the outer vertical edges of the raster.
Although a circuit using discrete components can generate the drive waveform required by the East-West amplifier, it is customary to employ a specialised geometry-correcting integrated circuit (IC). Such ICs provide a close approximation to the requirement of most picture tubes, which can be pre-set within close tolerances using memory registers within the IC. Both the DC and AC components of the drive waveform are set and stored for each display format (aspect ratio selection). However, there is now a range of picture tubes with more extreme geometric correction requirements, which are not met by existing or currently proposed geometry-controlling ICs.
This problem, which is referred to as xe2x80x9cDynamic East-West correctionxe2x80x9d, arises from the change in the raster shape and size that arises from the variation in the Extra High Tension (EHT) potential as a function of the beam current. The beam current varies strongly in response to the picture content on the screen.
Within the existing ICs, this correction is applied to the East-West drive in response to a signal voltage derived from a beam current detection circuit. This voltage represents the variation in EHT potential from which the required correction can be computed. This correction ensures that the display does not xe2x80x9cbreathexe2x80x9d during changes of picture content. In the present ICs this correction does provide an accurate correction of the DC (width) component, but the AC component is not corrected dynamically.
The present invention seeks to provide automatic dynamic correction of the AC component of the drive signal to the East-West amplifier in response to the normal EHT variation.
According to the invention, an electronic circuit is provided for controlling the AC component of the drive signal to an East-West amplifier in a picture tube horizontal scanning system, the circuit comprising an input for receiving a signal related to the beam current of the tube, an output for connection to the input of the amplifier, a capacitive means coupled to the output of the electronic circuit, and an active control device for controlling the capacitive means in response to the signal at the circuit input.
The circuit of the invention is able to achieve continuous variation of the AC component correction, responding accurately to the beam current information, and producing acceptable raster shape at all levels, whilst adding minimal cost.
This may be implemented by application of a large capacitor to the input port of the East-West amplifier. An active control device, such as a semiconductor xe2x80x9cresistancexe2x80x9d, is driven from the beam current information to control the effect of the capacitor. The capacitor is preferably connected in series with the active control device.
The proposed circuit may be applied to a system set up initially for a perfect raster at maximum EHT (i.e. low beam current condition with a crosshatch test pattern). In this case, as the EHT voltage falls the parabolic correction must be reduced to counteract the barrel distortion that results and the circuit aims to fulfil this function.
Preferably, the active control device is a bipolar transistor. It may be a NPN transistor connected between the capacitive means and ground. In particular, a close approach to the required control characteristic may be achieved by using a NPN transistor with its emitter grounded and its collector connected to the capacitor. In this unconventional mode, the input base current controls the effective resistance at the collector. The sense of the control signal for the NPN transistor may be of opposite polarity to that of the beam current signal and in that case, polarity inversion means may be connected between the circuit input and the transistor. This can suitably be provided by a single amplifier stage, which can be configured to form a high impedance interface, as well as setting the operating range of the circuit to model the required control characteristic over the range of operating currents for a particular picture tube.
Alternatively, PNP transistor or a FET may be employed as the active control device. In a PNP based configuration, the transistor may be connected between the capacitive means and a low impedance power supply. In a preferred arrangement, the collector of the transistor is connected to the capacitive means and its emitter connected to the power supply.
Other arrangements of the active control device and capacitive means besides those described above may be used. For example, with a NPN transistor, the capacitive means may be connected between its emitter and the East-West amplifier with its collector connected to ground, or its emitter may be connected to ground via the capacitive means and its collector connected to the amplifier. In the latter case, it may be appropriate to connect a resistor between the emitter and base of the transistor to control uncertainty due to the current gain of the transistor. Similarly, with a PNP transistor, the capacitive means may alternatively be connected between the emitter of the transistor and the low impedance power supply, with its collector connected to ground, for example.
Preferably, means for controlling the operating characteristics of the circuit are provided between its input and the active control device, such as the amplifier stage noted above in relation to the NPN based embodiment.
In some circumstances, the effect of the circuit is not symmetrical at the upper and lower corners of the image displayed on the picture tube. This is an inherent feature of the combination of a capacitor and resistor in series. This problem may be overcome by applying a sawtooth waveform to the control signal at the base input of the transistor. The circuit may therefore include a second input for receiving a waveform to improve the symmetry of the control applied by the circuit. This has no effect in the low-current condition when the transistor is turned off and a beneficial effect at other times. In a NPN transistor based configuration the waveform is preferably a negative-going field sawtooth, whilst where a PNP transistor is used as the active control device of the circuit, the waveform is preferably a positive-going sawtooth.